MySQL SQLLesson 33Free preview

MySQL Self Join

This lesson is part of MySQL · All levels

Summary

Self join is the merging of a table with itself, which is usually done by giving two different aliases to the same table. This technique is used when a table represents hierarchical relationships, for example employees with superiors who are also employees, or categories with category masters. Although the table names are the same, both aliases are required for MySQL to distinguish between the left and right sides.

Key points

Self join uses the same table twice with two different aliases
The ON condition compares the column to itself, for example e.id = m.id_manager
Suitable for hierarchical data such as top-down or parent-child categories
Self join makes special syntax, just the usual use of INNER JOIN

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